I did not know this route had existed until recently when I was sent the below photo of an AD sign showing the Alt-1 shield. I am yet to find a map that acknowledges its existance, presumably because of its very short length through Hornsby. My educated guess would be that Alt-1 was commissioned around 1973 or 19744 (when State Routes and Freeway Routes were introduced and not long after NAASRA adopted the guidelines for signing National Routes). I believe it would have survived until the F3 opened in March 1989, although perhaps signage was steadily disappearing as was the case with Alt-1 in Wollongong. State Route 83 now traverses the former route of Alt-1.

Photos of Alt-1 at Hornsby

AD sign showing Alt-1 as the current SR83 route through Hornsby (George St & Jersey St North). Thanks to Paul Rands for finding this photo in an old textbook from Model Farms High School! Taken c. 1977.